Adding PHP 8.2 in travis tests

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Phew, getting php 8.2 working in my plugin’s travis tests required quite a bit of trial & error in travis.yaml, but 8 commits later I won 😉

The solution was in these extra lines to ensure libonig5 was installed;

matrix:
  include: 
  - dist: focal
    php: 8.2
    env: LIBONIG_INSTALL=1

install:
  # for PHP 8.2 install libonig
  - if [ -n "$LIBONIG_INSTALL" ]; then sudo apt-get install libonig5; fi